1. Field of the Invention
This invention is generally related to a system for connecting a riser between a floating vessel and a subsea structure. More particularly, the invention provides apparatus and method for running a production riser to a subsea wellhead that reduces the size of the buoyancy air tanks in a deep draft caisson vessel and therefore the overall vessel size.
2. General Background
The ongoing search for hydrocarbons in offshore locations to satisfy the worlds increasing need for energy has led to the need to drill and produce these hydrocarbons in increasingly deeper waters. This has motivated the development of systems for drilling and producing hydrocarbons in ever deeper waters. Such systems have included the tension leg platform in which a buoyant structure is placed on the surface and anchored to the sea floor through pipes or tendons. The tension leg platform included means for ballasting and deballasting the platform to adjust the distance between the tension leg platform and the sea floor anchors for the tendons. This allows the tendons to be tensioned and act as a semi-rigid structure for maintaining the tension leg platform in position.
A second system developed for deep water drilling and production has been the spar or deep draft caisson vessel. This system uses in effect a long tube with a large sealed annular section that is positioned vertically. The sealed annular section of the deep draft caisson vessel includes a plurality of chambers that may be filled with water or air to control the buoyancy of the structure. A large central bore allows the positioning of well slots therein. As in the tension leg platform design, tubular members or risers as generally known in the industry extend from the sea floor to the surface structure.
It is important to minimize the size of the well slots and surrounding support structure to reduce the costs of the spar or deep draft caisson vessel. The current invention does this by reducing the through bore requirement in the buoyancy air tanks attached to the upper end of the riser and through which the riser is run. This is done by using an end connection member on the riser and providing a second hydraulic connector on the sea floor which can lock on the end connection member when it is deployed.
2. Description of Related Art
U.S. Pat. No. 4,673,041 to W. H. Turner et al. shows a hydraulically actuated connector used for well servicing on offshore wellhead systems.
A riser guide and support mechanism for use with a spar type floating vessel is disclosed in U.S. Pat. No. 6,176,646 B1 to L. D. Finn et al.
U.S. Pat. No. 6,193,441 B1 to E. A. Fisher shows an emergency dump apparatus for buoyancy air tanks.
The present invention comprises a riser with an end connection member that allows a smaller diameter through bore to be used in the buoyancy air tank through which it is run. The system is especially useful in a spar type structure or a deep draft caisson vessel that uses large diameter air tanks to tension the riser. A subsea structure such as a template or wellhead base is positioned on the sea floor and anchored thereto. A subsea wellhead housing is affixed to the subsea structure. A hydraulically actuated connector is sealingly connected to the subsea wellhead housing and a second hydraulically actuated connector is positioned above the first hydraulically actuated connector. The second hydraulically actuated connector may be connected to the first hydraulically actuated connector by conventional means as bolting or clamping or may be integrally formed together. The second hydraulically actuated connector is positioned facing upward to receive the aforementioned end connection member on the riser and lock thereon.
The floating vessel or deep draft caisson vessel positioned above the subsea wellhead includes a plurality of well slots. The well slots have air tanks or cans positioned therein with the air tanks secured together in end to end arrangement. Each of the well slots include guide sleeves positioned vertically along the well slot to restrain movement of the air tanks. The air tanks have a through bore through which the riser may pass. A stem joint extends from the top of the uppermost air tank. A riser stop is positioned on the stem joint and coacts with the lower framework of the deep draft caisson vessel to limit upward movement of the air tanks when deballasted.
Alternative embodiments are shown with mechanical connectors replacing the hydraulic connectors. Electrical connectors also are envisioned. Additionally, an embodiment with the lower connector integrally formed with the wellhead housing is shown.
A wellhead housing and christmas tree are connected to the upper end of the riser and rest on stem joint to allow tensioning of the riser as described. A method of operation is also described and claimed.
A principal object of the present invention is to provide an apparatus that reduces the size of the air tanks used on a deep draft caisson vessel and thereby allow closer positioning of the well slots and overall reduction in size of the deep draft caisson vessel.
These with other objects and advantages of the present invention are pointed out with specificness in the claims annexed hereto and form a part of this disclosure. A full and complete understanding of the invention may be had by reference to the accompanying drawings and description of the preferred embodiments.